The present invention relates to a scanning near-field optic/atomic force microscope for observing the topography of a substance to be investigated, by making use of an atomic force acting between substances, and at the same time for observing the optical property of a microscopic region of the investigated substance by a probe consisting of a light-propagating body.
Atomic force microscopes (AFMs) are capable of accurately observing the topography of the surface of a sample, irrespective of whether the sample is conductive or not, in contrast with scanning tunneling microscopes (STMs) and, therefore, AFMs are widespread. Atomic force microscopy is a measuring method utilizing the fact that a spring element supporting a measuring probe is deflected by an atomic force acting between a sample and the measuring probe.
In an attempt to measure the optical characteristics and the topography of a sample, a probe consisting of a light transmissive medium having a sharp front end was brought close to the sample to be investigated such that the distance between them was less than the wavelength of the light. Also, some near-field optical microscopes have been proposed. In one of these proposed optical microscopes, laser light is directed from the rear side of a sample such that the light is totally reflected by the rear surface of the sample. Evanescent light leaking from the front surface of the sample is detected by bringing the front end of an optical fiber probe close to the surface of the sample, the probe being equipped with a fine-motion mechanism. The topography of the surface is observed in the way that the probe is scanned horizontally and vertically so as to detect constant evanescent light, or the probe is scanned horizontally so as to measure variations in the intensity of the evanescent light.
In another proposed apparatus, the front end of an optical fiber probe is held vertical to a sample. The front end is vibrated horizontally over the surface of the sample to produce friction between the sample surface and the front end of the probe, thus resulting in vibrations. Variations in the amplitude of the vibrations are detected as deviations of the optical axis of laser light which is emitted from the front end of the optical fiber and transmitted through the sample. A fine-motion mechanism is actuated to move the sample so that the distance between the front end of the probe and the sample surface is maintained constant. The surface topography is detected from the intensity of the signal applied to the fine-motion mechanism. Also, the transmissivity of the sample for the light is measured.
In a further proposed apparatus, a glass capillary having a hook-shaped front end portion is used. A fluorescent material is loaded into the tip portion of the capillary. A reflecting sheet for optically detecting deflections of the probe is installed on the rear side of the capillary, i.e. on the opposite side of the front end of the hook-shaped portion. Light is emitted from the back side of the sample and transmitted through the sample. This causes the fluorescent material at the front end of the probe close to the sample to emit light, which is transmitted through the sample. This light is detected on the rear side of the sample. In this way, the sample is investigated by atomic force microscopy. At the same time, the transmissivity is measured.
A still other proposed apparatus uses a probe consisting of an electrically conductive and light transmissive medium as an STM probe so as to measure the optical characteristics of the sample simultaneously.
The prior art AFM and STM techniques are adapted for observation of surface topography but are incapable of measuring the physical and chemical natures of a sample. A method of using light as a means for observing these properties of a sample is contemplated.
In some apparatuses of near-field optical microscopes, an evanescent light is used as the above-described light. In such an apparatus, light intensity is used as information regarding the direction of height. Therefore, it is impossible to separate variations in the light intensity in the direction of height from light intensity variations due to absorption of light into a sample. Hence, it is difficult to use this apparatus as a means for measuring the physical and chemical properties of a sample. Where the sample surface is greatly uneven, light may not be totally reflected by the rear surface of the sample but be transmitted through it. Transmitted light rays may interfere with each other on the surface of the sample, thus hindering measurement.
In an apparatus for dividing a topography image and optical characteristic image of a sample to simultaneously measure them, a non-contact type atomic force microscope or shear force type atomic force microscope wherein a probe which is a light propagating body has been proposed. In such apparatuses, where light is constantly radiated onto the sample, there is the problem that detection is impossible where the light intensity to be detected is low. Also, where the sample is a fluorescent material, there is the possibility that the sample will receive damage from the constant light radiation. Further, in a non-contact type atomic force microscope which vibrates the probe vertically with respect to the sample, because the spot of the light radiated when the probe distal end is distanced from the sample surface during the phase of a vibration cycle is broadened when it arrives at the sample surface, the light intensity and resolution thereof are reduced. In a shear force type atomic force microscope where the probe is vibrated horizontally with respect to the sample, the light radiated when the probe distal end is displaced from the vibration center during the phase of a vibration cycle is displaced from the measurement point, therefore resolution is reduced. Where the sample is a fluorescent material there are cases where, when a metal is brought into contact with the fluorescent body, the fluorescent energy thereof is absorbed by the metal and fluorescence is not produced, and where light propagating probes having a metal covering over the distal portion thereof are used, cases where light radiation is necessary during the phase where the probe distal end is distanced from the sample surface must be considered.